This invention relates to devices used to couple electrical signals into living tissue and especially to those devices which utilize magnetic stimulation to induce a current in human or animal nerve tissue.
The use of magnetic stimulation in clinical neurophysiology has been shown to be a useful research and diagnostic tool in medicine. In recent years medical researchers have found that it is possible to stimulate selected nerves with magnetically induced signals from coils carrying high current pulses. This stimulation means was found to be safe, painless, non-invasive, and easy to administer. The alternatives to magnetic stimulation are to use needle electrodes to penetrate the skin, or surface electrodes on the skin to apply high-voltage shocks through the body. Until the design of the device described herein, the main disadvantage of magnetic nerve stimulation was its imprecision.
Present magnetic stimulator coils are neither capable of precise application of stimulus nor usually oriented for proper stimulus of nerves. The most popular design uses circular coils, which are typically applied flat on the skin for stimulating the nerves. This results in an imprecise, broad stimulus which is not sufficient for accurate nerve measurements such as nerve latency. Use of existing coils results in imprecise knowledge of where primary stimulus signal is being applied, and signals which contaminate readings by stimulating adjacent tissue.
Present magnetic stimulus waveform generators, likewise, are not capable of inducing broad monopolar or symmetric bipolar signals. The current methods to generate the high current pulses for nerve stimulation usually consist of discharging a capacitor through a coil, in a short burst through a switch. FIG. 1 illustrates a typical prior art circuit. This circuit generates a bipolar current waveform in the coil (inductor) if the L-C circuit is underdamped, and a bipolar induced waveform in the nerve. Even when a critically damped or slightly overdamped waveform is used, a bipolar induced signal results. This occurs because the induced signal is the first derivative of the current waveform in the coil. FIGS. 2A and 2B illustrate a critically damped current pulse and its resultant induced voltage.
What is needed is a magnetic stimulator device designed to improve the knowledge of where the stimulus is being applied, improve the signal intensity for a given coil inductance, reduce the volume of tissue receiving the strongest stimulus signals, and reduce the contaminating affects of unwanted signals. Also needed are magnetic stimulation devices and methods for providing single polarity, precise duration, precise magnitude, induced signals for improving nerve conduction measurement.
With these improvements the technology of magnetic stimulation of body parts will be made much more exact and be able to compete with the non-magnetic techniques in stimulus accuracy, while retaining all the inherent advantages of magnetic stimulation.